Exposure to the opioid antagonist naltrexone throughout gestation alters postnatal heart development

The influence of endogenous opioid blockade by naltrexone during prenatal life on postnatal heart development was studied in rats. Pregnant Sprague-Dawley rats received daily injections of 50 mg/kg naltrexone (NTX) or saline throughout gestation; offspring were cross-fostered at birth to mothers not receiving NTX. In general, NTX-treated offspring weighed more than controls throughout preweaning life, whereas heart weights were often increased from age-matched controls up to 35 days. Biochemical analyses of nucleic acids and protein demonstrated that DNA and protein content were increased throughout development in NTX-treated animals relative to controls. Morphometric analyses revealed increases in total area of the heart and myocardial area in NTX-exposed rats relative to control levels. These data suggest that endogenous opioids function to regulate cardiac growth during the prenatal period, and that disruption of this process has long-term implication for cardiac biology.